Many systems use light emitting diodes (LEDs) to generate light. Often times, multiple LEDs are coupled together to form a string of LEDs, and a switched mode regulator generates a supply voltage for the LED string. To control the amount of current flowing through the LED string, a resistive sense element is typically coupled in series with the LED string. The current flows through the resistive sense element, generating a voltage that can be compared to a reference voltage. The regulator can use the result of the comparison to adjust the voltage supplied to the LED string. The regulator often uses a pulse width modulation (PWM) control technique to adjust the voltage supplied to the LED string.
This type of system can suffer from various shortcomings. For example, fixed-frequency PWM control techniques often require loop compensation to achieve a stable control loop. However, this typically limits the system response and increases circuit complexity. Also, a resistive sense element coupled in series with the LED string dissipates power during the LED string's entire operating period. This reduces system efficiency and can result in major power losses when driving high-power LEDs.